The incidence of hospitalization for the removal of urinary calculi, commonly referred to as kidney stones, has been estimated to be as high as 200,000 cases per year. Stones which are quite small may in some cases be passed without hospitalization. However, in the remainder, the kidney stones become impacted in the ureter, a muscle tube joining the kidney to the bladder. An impacted kidney stone is a source of intense pain and bleeding, a source of infection, and, if the stone completely blocks the flow of urine for any extended length of time, can cause the loss of a kidney. Small stones which are lodged in the lower third of the ureter can be removed non-surgically by using a technique employing the well known Dormia stone basket. However, basket removal of a kidney stone usually fails if the stone is lodged in the upper ureter, is impacted, or is larger than one centimeter in diameter. In these cases, the only procedure by which the stone could heretofore be removed was through a serious surgical operation called a ureterolithotomy.
More recently, however, a non-surgical method for removing kidney stones has been developed. In this technique, the kidney stones are subject to a vibratory action, such as that provided by an ultrasonically vibrated wire probe. In one such apparatus, a catheter is placed cystoscopically at the site of the stone in the urinary tract, after which a long wire probe is passed through a lumen of the catheter and is brought into contact with the stone. The wire probe is attached to the ultrasonic transducer which, when energized, sets the wire probe into lateral and longitudinal vibrations. The vibrational energy is transmitted by the wire probe to the stone, causing it to fracture into small fragments which the patient can then pass spontaneously after withdrawal of the catheter.
While such apparatus can perform its intended function, a severe limitation has been found to exist due to slippage of the thin wire probe off the kidney stone and its lodgement between the stone and urinary tract. Attempts to ameliorate this situation by providing a wider tip cutting surface has resulted in premature breakage of the wire probe at the tip exiting the catheter near the kidney stone. This occurs because the vibrational energy, typically 20 KHz in frequency, in conjunction with the added cantilevered mass of the larger cutting area, causes the wire probe to snap off due to metal fatigue after 10-20 seconds of operation. This has been found to be insufficient time to complete the disintegration procedure, as well as dangerous because it leaves metal fragments in the urinary tract. A clear need, therefore, exists to prolong the operational life of the wire probe for a length of time adequate for completion of the procedure. A desirable time frame is typically one minute, which is substantially greater than that provided by known prior art apparatus.